Circuit Description

A/C Compressor Clutch

Whenever the ignition switch is in the "ACCY" or "RUN" position, battery voltage is applied through the Gauge Fuse and Audio Alarm Module (Integration Relay) to the coil of the Heater Relay. With the BLOWER SWITCH in any position but "OFF," the Heater Relay coil is provided with a ground through the BLOWER SWITCH at JUNCTION BLOCK 4. With a ground provided, the Heater Relay is energized, the relay contacts close, and battery voltage is applied through the Heater Circuit Breaker, the closed Heater Relay contacts, and the A/C Fuse to the coil of the A/C Clutch Relay.

When the proper inputs are received, the A/C AMPLIFIER provides a ground for the coil of the A/C Clutch Relay. This set of inputs includes the following signals:

^ A/C request signal at cavity 4 indicating that the A/C SWITCH has been turned to "ON."
^ THERMISTOR signal at cavities 6 and 12 indicating that the evaporative temperature of the refrigerant is 1°C (34°F) or above.
^ A/C DUAL PRESSURE SWITCH signal indicating that refrigerant pressure is between 206 kPa (30 psi) and 2648 kPa (384 psi).
^ Engine speed signal from the igniter at cavity 14 that indicates that the engine is running at an acceptable idle speed. If engine idle speed drops, the A/C AMPLIFIER will provide a ground for the A/C IDLE-UP SWITCHING VALVE. The A/C IDLE-UP SWITCHING VALVE reroutes engine intake manifold vacuum and increases engine idle speed.

When the A/C AMPLIFIER provides a ground for the A/C Clutch Relay, the relay is energized, the contacts close, and battery voltage is applied through the ECM-IG Fuse and the closed contacts of the A/C Clutch Relay to the A/C COMPRESSOR CLUTCH. With voltage applied, the A/C COMPRESSOR CLUTCH engages and A/C System operation begins. When the A/C Clutch Relay is energized, the ENGINE CONTROL MODULE is also provided with a signal that indicates that the A/C COMPRESSOR CLUTCH is engaged.

Blower

Battery voltage is applied at all times to the Heater Circuit Breaker. Battery voltage is also applied to the coil of the Heater Relay whenever the ignition switch is in "ACCY" or "RUN." When the BLOWER SWITCH is turned to "LOW," the coil of the Heater Relay is grounded, the relay is energized, and battery voltage is applied through the Heater Circuit Breaker and the closed contacts of the Heater Relay to the BLOWER MOTOR. With the BLOWER SWITCH in "LOW," current must travel all the way through the resistive element in the BLOWER RESISTOR before the BLOWER MOTOR can receive a ground at JUNCTION BLOCK 4. Therefore, the reduced voltage potential caused by the BLOWER RESISTOR maintains the BLOWER MOTOR at "LOW" speed.

When the BLOWER SWITCH is in the M1 position, the BLOWER MOTOR'S ground path passes only partially through the BLOWER RESISTOR and then through the BLOWER SWITCH before reaching ground at JUNCTION BLOCK 4. Since less of the BLOWER RESISTOR is in the circuit, the resistance in the circuit is lower and, in turn, the voltage potential of the circuit is higher. Therefore, the BLOWER MOTOR operates at a higher speed than it does with the BLOWER SWITCH in the "LOW" position.

M2 operation is identical to M1 operation with the exception that even less of the BLOWER RESISTOR is in the circuit. Therefore, BLOWER MOTOR operation in M2 is faster than it is in M1.

With the BLOWER SWITCH in "HIGH," the ground path of the BLOWER MOTOR bypasses the BLOWER RESISTOR entirely and passes directly through the BLOWER SWITCH to JUNCTION BLOCK 4. Therefore, the resistance in the ground path is nearly zero and the BLOWER MOTOR operates on full battery voltage. This negligible resistance and, in turn, higher voltage potential causes the BLOWER MOTOR to operate a full speed.